Intensity control system for a particle beam device



Dec. 24, 1968 R. R. BARBER ETAL 3,418,520

INTENSITY CONTROL SYSTEM FOR A PARTICLE BEAM DEVICE 2 SheetsSheet 1Filed Dec. 21, 1966 Dec. 24, 1968 R. R. BARBER x-:TAL

INTENSITY CONTROL SYSTEM FOR A PARTICLE BEAM DEVICE 2 Sheets-Sheet 2Filed Dec. 2l, 1966 II IIJ IFIER 44 REFERENCE SIGNAL AMPLIFIERDIFFERENTIAL Fls TARGET CURRENT SIGNAL IU Fmsr LENS CURRENT BY @all 5MMATIURNEY United States Patent O 3,418,520 INTENSHTY CGNTROL SYSTEM FOR APARTICLE BEAM DEVICE Robert Russell Barber and Karl Heinz Loefller, San`lose,

Calif., assignors to international Business Machines Corporation,Armonk, NX., a corporation of New Yori:

Filed Dee. '21, 1966, Ser. No. 603,556 1t) Claims. (Cl. 315-31) ABSTRACT0F THE DISCLOSURE A control system for regulating the intensity of abeam spot of a particle beam device such as an electron beam columnwherein the magnication of a variable magnication lens is adjustedresponsive to the measured beam current to regulate the beam spot sizeat an aperture plate thereby to regulate the current density of the beamand thereby the beam current passing through the aperture.

This invention relates to a control system for regulating the intensityof the beam spot of a particle ybeam device such as an electron beamcolumn for use in data recording systems, wherein the magnification of avariable magnication lens is adjusted responsive to the beam intensityat a target to control the beam spot size at a downward positionedaperture plate thereby to regulate the particle density of the beampassing through the aperture.

In data recording, a particle beam is scanned across a memory element ortarget to record a pattern or image for future reference. The beam maycomprise such particles as electrons, ions, or photons which areaccelerated and focused to a spot size having a small cross-sectionaldiameter for use in writing on the target. For instance, electron beamsnow are being used to inscribe on silver halide film certain patternsindicative of data in digital form for use in data handling systems.

To record the data, the beam either is ymodulated in a pre-determinedmanner or is deflected to inscribe patterns representing the data ontothe memory element. Since the beam intensity frequently is modulated inresponse to the data being recorded, it is important that the intensityof the beam prior to such modulation be controlled closely so that theonly change in the beam is one actually responsive to the data beingrecorded.

In the past, various means have -been used to control the beam intensityin such devices. For instance, if the beam is originated at a heatedcathode, the cathode current can be regulated in an attempt to maintainthe beam at a constant intensity. However, the response of such a systemis usually not sufiiciently regulated and frequently is too slow, dueprimarily to the thermal inertia of the cathode, for use in datarecording systems. Another method previously used has been to deflectthe beam to one side of its original axis to cause a portion thereof tostrike an opaque member positioned adjacent to the beam axis. This sidedeflection prevents the cutol'rr portion of the beam from striking thetarget, thereby lowering the overall intensity of the beam at thetarget, This method resuits in generating a beam of irregularcross-section which frequently is unsuitable for recording data ontomemory elements. Additionally, when the beam is shifted off axis in thismanner to reduce the beam intensity, it becomes more difficult tocontrol the alignment of the scan pattern of the beam with the memorystorage lplane.

In accordance with the present invention, the current intensity of aparticle beam spot is regulated by sensing the beam intensity at thetarget and by adjusting the focal power of a variable magnificationlens, positioned between the beam source and an opaque aperture-formingPatented D'ec. 24, 1968 member, in a manner responsive to the differencebetween the measured intensity of the beam spot and the desiredintensity of the beam spot. The cross sectional current density of thebeam is varied by changing the beam spot size or diameter at theaperture-forming member such that more or less of the outer fringes ofthe beam are intercepted thereby regulating the intensity of the beampassing through the aperture and adjusting the overall intensity of thebeam reaching the target. A further embodiment of the invention combinesthe before-mentioned intensity control with a second lens in theparticle beam device having a magnification which is varied responsiveto the focal power adjustment of the lens for maintaining the overallmagnification of both lens, and therefore of the column, constant.

One object of this invention is to provide an improved control forregulating the intensity of the beam spot of a particle beam device.

Another object of this invention is to regulate the spot current of aparticle beam device quickly, accurately and with little effect on theoverall functioning of the beam.

A more detailed object of this invention is to regulate closely andcontinuously the spot current of a particle beam device, in a manner tominimize any adverse effects on the operation of the beam device as thebeam current is regulated.

Other and further objects, features and advantages of this inventionwill be apparent from the following particular description of preferredembodiments of the invention as illustrated in the accompanyingdrawings, in which:

FIGURE l is a perspective view of an electron beam column of a type inwhich the subject invention can be employed;

FIGURE 2 is a `side cross-sectional view of an aperture assembly used inthe electron beam column shown in FIGURE 1;

FIGURE 3 illustrates the column of FIGURE 1 in diagrammatic form with aschematic drawing of one embodiment of the beam intensity controlsystem; and

FIGURE 4 illustrates graphically the manner in which the beam targetcurrent varies as the first lens current is adjusted in accordance withthe invention.

In FIGURES l and 3 are shown an electron beam column S representing onetype of particle beam generating device in which the subject inventioncan be employed. In the present instance, the column is adapted for usein data recording wherein the beam is directed onto a target or memoryelement 9 for recording an image. Usually this target is removable fromthe column. The column comprises an elongated tubular housing 1t) with acathode 11 supported adjacent an anode 12 in one end of the columnserving as an electron source for emitting a beam of electrons of apreselected magnitude of intensity for passage along the column axis 13.

The electron beam is focused to a small spot size by being passedthrough the magnetic fields of axially spaced electromagnetic lenses 14,15 and 16 positioned in spaced relationship along the axis 13 within thehousing 10. Each of these lenses includes a pair of polepieces 17 and 18which transmit the magnetic flux generated in the respective electriccoils 14a, 15a and 16a to a point closely adjacent to the beam axis 13.A non-magnetic spacer 19in each lens maintains the ends of thepolepieces adjacent the axis in axially-spaced relationship. The lenses14 and 1S also include polepiece extensions 2t) and 21 separated by anon-magnetic member 22 and held in a non-magnetic cylinder 23, whichextensions receive the magnetic flux of the respective lens andcooperate to form the lens magnetic gap at a position closely adjacentto the beam axis 13.

By a proper energization of the lens coils 14a, 15a and 16a, a magneticfield is formed which deflects the electrons of the beam back towardsthe axis 13 for focusing the beam to a small spot size. The beamthereafter is passed through an aperture assembly 24a, 2411 and 24e(FIGURES 1 and 2) positioned downstream of the respective lens. As shownin FIGURE 2, each aperture assembly includes a housing 2S held withinthe non-magnetic cylinder 23. Within the housing is positioned a support26 for mounting a beam-opaque aperture plate 27a, 27b and 27C in whichis formed a small aperture 28a, 28h and 2SC, respectively, at a positioncoinciding with the beam axis. By first passing the bear through themagnetic field of each lens and subsequently through the cooperatingaperture assembly, the beam is formed to a very small cross-sectionaldiameter or spot size suitable for writing data onto the target 9 at avery high density. The beam is focused in the plane of the target 9 byenergizing a focusing coil 30 positioned adjacent the lens 16. Byproperly adjusting the magnitude of electric current supplied to thisfocusing coil (in a manner not specifically shown herein, but which iswell known in the art), the focal power of lens 30 is varied foradjusting the image size at the plane of the target. Additionally, anannular-shaped deliection coil 31 is provided which, when energized,serves to deect and scan the beam across the memory element forrecording the data thereon.

To modulate the beam responsive to the data being recorded, a pair ofelectrostatic deliecting plates 32 and 34 are positioned respectively toeach side of the axis 13. By energizing these plates, the beam isdeflected suiciently to become misaligned -with the aperture 28 of thedownstream positioned aperture assembly 24C thereby effectively shuttingoff the beam. One method of recording data onto the target 9 is bysuperimposing a sine-wave signal onto the normal signal supplied to thedeflecting coil 31 such that the beam is deflected back and forthrapidly at a direction perependicular to the scan direction as the beamis scanned in a manner to paint an exposed area onto the target. Forinstance, a one commonly used in digital data recording can berepresented by an exposed or painted area followed by an unexposed area,while a zero can be represented by an unexposed area followed by anexposed area. Thereafter, the reader can detect the ones and zeros bydetecting the exposed and unexposed areas and their sequence ofoccurrence.

However, to make such an encoding process valid, it can be seen that theintensity of the beam spot must vbe controlled closely. Otherwise, withthe variance of the beam intensity, a painted area might remainunexposed if the beam spot intensity was lessened substantially.Conversely, if the spot intensity was increased sufficiently, the sizeof the exposed area might be enlarged sufficiently to expose an adjacentarea which is desired to be unexposed thereby rendering the recordingmethod invalid. Various external factors can cause the spot intensity tochange such as changes in the filament current, a reduction in filamentsize during the life of the filament, etc.

The overall purpose of this invention is to control closely the spotintensity of a particle beam column such as the one heretofore describedso that the recording function is unaffected by undesired fluctuationsin the intensity of the beam. In accordance with the present invention,the intensity of the beam is controlled by generating a first signalresponsive to the measured spot intensity at the target and, bycomparing the measured beam intensity signal Iwith a second signalindicative of the beam intensity desired, a differential signal isgenerated which is used for adjusting the magnification of one of theelectro magnetic lenses thereby to vary the beam diameter at thecooperating sperture assembly to control the density of the electrons ofthat portionof the beam passing through the aperture for regulating theoverall intensity of the beam striking the target.

While the invention is described for use with an electron beam device,it is realized that the intensity of other types of beam devices such aslaser beam generators could be controlled in a similar manner and withequally beneficial results. Naturally, the lenses of such other beamdevices would of necessity be appropriate for regulating themagnification of the beam whose intensity is being controlled.

A preferred embodiment of the invention is shown in FIGURE 3 wherein theintensity of the beam is detected by a detector 37 positioned adjacentthe target 9, with fa signal responsive to this measured intensity beingfed to a level detector 38. The signal from the level detector is fed toa differential amplifier 39 along with a reference signal for generatinga differential signal indicative of the difference between the measuredbeam intensity and the desired beam intensity. Thereafter, thedifferential signal is fed to a lens current control 40 for varying themagnification of the lens 14 thereby to vary the beam image size at theadjacent aperture plate 27 of the aperture assembly 24a. In this manner,the current density of that portion of the beam passing through theaperture 28a is varied to change the overall intensity of the beampassing through the aperture and thereafter striking the target 9.

In FIGURE 3, the control is illustrated in diagrammatic form in which analternating current signal is generated responsive to the measuredintensity of the spot. To accomplish this, a target 9 is used having aseries of adjacent and alternately positioned opaque `and transparentportions such as is formed by the series of opaque lines 41 extending ina direction perpendicular to the scanning direction of the beam. As thebeam is scanned across these lines, it alternately strikes and isprevented from striking the detector 37. This detector is one of manytypes for generating a signal responsive to the beam current strikingit. One example of such a detector is a P/N junction of standard designhaving an electrical impedance proportional to the strength of the beamstriking it. Thus, an alternating current signal is imposed on aconstant magnitude current being passed through the detector as the beamintermittently strikes the detector which signal has a peak valueresponsive to the measured intensity of the beam. The AC signal is fedthrough a conductor 42 to a level detector 38. The level detectorincludes an AC amplifier 44 of standard design which amplifies themeasured-intensity signal and feeds it to a peak voltage detector 45.The peak voltage detector is one of several types commercially availablefor generating a direct current signal responsive to the peak magnitudesof the measured intensity signal received from the amplifier 44. While aDC signal could be generated responsive to the beam intensity sensed bythe detector 37, by excluding the special target 9 having the opaquelines 41, the overall drift of this control is minimized by utilizing anAC signal which is amplified in the AC amplifier 44.

The amplified direct current measured-intensity signal is fed to thedifferential amplifier 39 along with a reference signal fed through theconductor 46. This reference signal is a direct current signalindicative of the intensity desired `at the detector 37 and can begenerated in any of many well known ways for achieving a direct currentsignal having a level which preferably can be adjusted to set the levelof intensity desired. The differential amplitier compares the peakvoltage DC signal and the reference signal to generate a differentialsignal which then is amplified and transmitted to a lens control 40.

It is necessary now to adjust the beam current in response to thedifferential signal generated by the differential amplifier 39 to avalue near that indicated by the reference signal received through line46. Thus, the lens current control 40 adjusts the focal power of theelectromagnetic lens 14 to change the image size of the beam at theaperture plate 27a of the aperture assembly immediately downstream ofthe lens. By varying the image size of the aperture plate, more or lessof the beam is allowed to pass through the aperture 28a thereby varyingthe overall beam current.

The lens control 40 comprises an analog storage device 48 which receivesthe differential signal and stores it until another signal is receivedfrom the differential amplifier. The storage device can be any of manywell known types such as a field effect transistor (not shown) havingthe bias voltage supplied by a large value capacitor which receives andis charged by the differential signal. From the analog storage device,the differential signal is fed 4to a first lens driver 49 whichcomprises a current source for supplying an electric current through theconductors 53 and 56 to the coil 14a of the lens 14 at a magnituderesponsive to the signal received from the analog storage device. (Itwill be noted that conductors 53 and 56 also connect With the coil 15a.The purpose of including the coil in the circuit will be explainedlater.) Thus, the intensity of the beam is measured and a signalgenerated responsive to the measured intensity. This measured-intensitysignal is peak detected and compared with a reference signal forgenerating a differential signal which then is supplied to the firstlens driver to adjust the magnitude of the current supplied to the lens14a for varying the focal power of that lens.

As shown in the graph of FIGURE 4, as the first lens current is varied,the target current signal is adjusted since the image size at theaperture plate 27a is changed as the magnification of the lens 14changes. Assume, for instance, that it is desired that the magnitude ofthe target current signal be at a level corresponding to point 50 on thecurve. However, the control just described senses a target currentsignal having a level corresponding to point 51 on the curve. With thefirst lens current being that indicated by the point 51, the outline ofthe beam is represented by the solid line 52 in FIGURE 3. It isnecessary, therefore, to reduce the target current or beam intensityfrom the value indicated by point 51 to that indicated by point 50. Fora smaller beam current, the magnification of the lens 14 is reduced toenlarge the beam diameter at the aperture 28a. Thus, the current densityof the lbeam at the aperture is reduced, and as a consequence a fewernumber of electrons are permitted to pass through the aperture 28a.

To reduce the magnification of lens 14, the lens control 40 receives asignal indicating that the first lens current should correspond to themagnitude indicated by point 50 on the curve, which indicates that thefirst lens current should be increased. As the first lens current isincreased, the beam shape is changed from that indicated by the solidline 52 to a shape corresponding to the dotted line 54 (FIGURE 3)thereby increasing the diameter of the beam at the aperture plate 27a.With the increase in diameter, the current density at the aperture plate27a is decreased since the current level of the beam remainssubstantially constant for periods of time substantially longer than ittakes for this control to react. Therefore, the total number ofelectrons passing through the aperture 28a is decreased and the targetcurrent assumes the value corresponding to point 50 on the curve.

It should be understood that before the control adjusts the first lenscurrent to a value to achieve the desired target current, the controlmay go through several cycles of readjusting the first lens current toobtain the proper target current. However, after a very few cycles, thefirst lens current will be readjusted to a value corresponding to thatof point 50 on the curve. Fortunately, the normal changes in beamintensity which this control is meant to correct are relatively slow inacting thereby allowing the control continually to maintain the beamcurrent at the desired value. Of course, as the reference signal ischanged in magnitude indicating a different desired target current, thepoint 50 Will assume a different value on the curve with the controlsystem thereafter readjusting the first lens current to correspond tothat desired target current signal. Also, while the outline of the beamis shown as a finite line, it is assumed that this is an outlinerepresenting the area of the beam having an electron density above agiven value.

As another feature of t-he invention, the magnification of the adjacentdownward lens is adjusted as the magnification of the first lens isadjusted for changing the beam intensity, thereby limiting any overalleffects on the functioning of the beam as the intensity is changed bythe heretofore described control. As can be seen in FIGURE 3, when themagnification of the lens 14 is changed to alter the shape of the lensfrom that indicated by the solid line 52 to that indicated by the dottedline 54, ordinarily the downstream image size at the aperture plate 27balso is changed from that indicated by the solid line 57 to thatindicated by the dotted line 58. This alternative embodiment of thisinvention involves feeding the first lens driver signal through theconductors 53 and .56 to the lens to change directly the magnificationof the lens 15 as the magnification of the lens 14 is changed. As can beseen in the drawings, as the magnification of the lens 14 is increasedfrom that indicated by the solid. line to that indicated by the dottedline, the electrons are passed through the aperture 28a at a greaterangle thereby altering the angle at which they approach the downstreamaperture plate 27b. By increasing the magnification of the lens 15, asthe magnification of lens 14 is increased, the image distance of lens 15is maintained substantially constant to decrease any overall effects onthe spot as the beam intensity is changed. The change in magnificationof lens 15 is made proportional to the magnification change in lens 14-and is effected by reducing the magnitude of the flux generated by theelectric current supplied to lens 15 by an amount proportional to theincrease of lens 14 current or vice versa depending on the direction ofchange of the magnification of the lens 14. One method of changing theflux level of coil 15a is by supplying the same current to that coil asis supplied through the conductors 53 and 56 to the coil 14a fortransmission through an oppositely wound section of the coil 15a tocounteract the normal fiux generated in lens 15.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art -that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention.

We claim:

1. In a beam device having a source for generating the beam having apreselected intensity magnitude, a first variable magnification lens andaperture forming plate assembly for focusing the beam, and la target forreceiving the beam in that order, a beam intensity control systemcomprising:

means for generating a first signal responsive to the beam spotintensity at the target, and

means for adjusting the magnification of the lens to vary the diameterof the beam at the aperture forming plate in response to said firstsignal to adjust the density of the beam thereby to set the beam spotintensity at the target to a predetermined magnitude.

2. A beam intensity control as defined in claim 1 wherein said variablemagnification lens includes a magnetic fiux generating coil and saidmeans for adjusting the lens magnification includes an electric currentsource for supplying electric current flow to said coil. at a magnituderesponsive to said first signal.

3. A beam intensity control as defined in claim 2 wherein said means forgenerating a first signal includes a beam detector for generating asecond signal responsive to the intensity of said beam; and

means for supplying a reference signal indicative of the beam intensitydesired, which reference signal is compared with said second signal togenerate said first signal.

4. A beam intensity control as defined in claim 1 including means formaintaining the beam focusing downward of said aperture forming plateassembly substantially constant as the magnification of said first lensis adjusted thereby to prevent the Operation of the beam from beingadversely affected as the beam intensity at the target is changed.

5. A beam intensity control as defined in claim 4 wherein said means formaintaining said beam focusing constant comprises a second variablemagnification lens with means for changing the magnification thereof inresponse to the changing of the magnification of said first lens.

6. A beam intensity control as defined in claim 5 whereinV saidsecondlens is4 positioned immediately downward of said first lens andthe magnification thereof is changed directly and proportionally as themagnification of said first lens is changed to vary beam intensity.

7. An electron beam column comprising:

a source for generating an electron beam having a preselected intensitymagnitude for passage along a predetermined column axis;

a target positioned along said axis from said source to be struck bysaid beam;

a beam-opaque aperture plate held between said source and target alongthe axis forming an aperture through which the beam must pass to reachthe target;

a variable magnification lens positioned to vary the image size of saidbeam at said aperture plate thereby to regulate the density of theelectrons of that portion of the beam passing through said aperture;

means for measuring the current intensity of the beam at the target; and

means for adjusting the magnification of the lens in response to themeasured beam current intensity thereby to adjust the current density ofthe beam passing through the aperture thereby to regulate the beamintensity to a predetermined magnitude at the target below thatpreselected intensity magnitude of the beam at said source.

8. An electron beam column as defined in claim 7 wherein said lensincludes a variable magnitude electric current source and an electriccoil connected to receive current from said source for establishing amagnetic fiux field intercepting said beam for focusing said beam atfocal points positioned in the proximity of said aperture plate, withthe precise focal point position of the beam being determined by a firstsignal in the form of an electric current being passed through said coiland the magnitude of said current being controlled in response to themeasured beam intensity.

9. An electron beam column as defined in claim 8 wherein said means formeasuring the beam intensity is actuated intermittently Yand includesYan analog storage Y' References Cited UNITED STATES PATENTS 6/1951Szegho et al 315-21 X 2/1953 Ellis 315-31 RODNEY D. BENNETT, PrimaryExaminer.

T. H. TUBBESING, Assistant Examiner.

U.S. Cl. X.R. 315-21, 22, 30

